The current invention relates to the rotor of a turbo-machine, such as steam turbine. More specifically, the current invention relates to an apparatus and method for preventing axial movement of a disc shrunk on the shaft of a steam turbine rotor.
The rotor of a steam turbine is typically comprised of a shaft having a plurality of blades affixed to its periphery. In addition, the shaft also features a disc, referred to as a "dummy" disc, which is unbladed. In one design, the dummy disc is shrunk onto the shaft so that it is held by an interference fit and radial pins near the downstream face. The dummy disc induces an axial force on the rotor which serves to balance the thrust which the steam expanding through the blading imposes on the rotor. As a result of differential thermal expansion between the disc and the shaft, as well as the centrifugal force on the disc, the interference fit holding the disc onto the shaft is reduced during operation. Temperature causes the disc to elongate upstream away from the pins. The interference fit then becomes tighter on the upstream face because the disc becomes umbrella-shaped due to the downstream face being hotter than the upstream face. Upon shutdown, the disc tends to thermally shorten, thus pulling on the pins while clinging to the rotor on the upstream face. Consequently, the dummy disc has a tendency to ratchet along the shaft during start-stop cycles. Such axial movement of the disc can result in the disc interfering with the stationary components attached to the steam turbine casing.
In the past, radially oriented retaining pins, which extended through holes in the dummy disc and the shaft, were used to prevent axial movement of the dummy disc along the shaft. However, the pins have not been entirely successful in restraining the discs and inspection has shown that, in some rotors, the dummy disc has moved axially as much as 0.150 cm (0.060 inch). Such axial movement has been accompanied by severe deformation of the radial pins, indicating that the pins lacked sufficient shear area to withstand the axial forces tending to move the disc.
Accordingly, it would be desirable to develop an apparatus having a large amount of shear area for preventing axial movement of a shrunk-on disc along a shaft. In addition, it would be desirable to provide a method of retro-fitting such an apparatus onto an existing steam turbine rotor. Moreover, since it is thought that differential motion between the disc and rotor at the upstream face of the disc must be eliminated to prevent the ratcheting motion of the disc, it would be desirable to retain the disc at its upstream face.